Wavelength filters are key components for wavelength division multiplexed (WDM) photonic networks. Potential applications for wavelength filters are demultiplexers, multiplexers, wavelength add/drop devices, noise filters and so forth. InP/InGaAsP is the material of choice for these filters because it allows for monolithic integration with photonic components such as transmitters, receivers, optical amplifiers, switches, etc. Among the various schemes proposed, the waveguide grating assisted coupler filter is a very attractive candidate due to its compactness, narrow bandwidth and tuneability. The device allows efficient wavelength selective energy transfer from one waveguide into a second waveguide by means of a common periodic structure, referred to as a grating. The wavelength .lambda. transferred by the coupler is defined by equation (1). ##EQU1## with waveguide propagation constants ##EQU2## and effective indices: N.sub.k ; and .LAMBDA..sub.g is the pitch of the grating. The `+` sign applies for backward coupling and the `-` sign applies for forward coupling gratings. This concept is well known (for example see: S. E. Miller, `Some Theory and Applications of Periodically Coupled Waves`, The Bell System Technical Journal, Vol. 8, September 1969; also see: D. Marcuse, `Directional Couplers Made of Nonidentical Asymmetric Slabs, Part II: Grating Assisted Couplers`, Journal of Lightwave Technology, Vol. LT-5, February, 1987), and vertically coupled devices were successfully implemented in the InP/InGaAsP material system (For example see: R. C. Alferness, et al., `Broadly Tuneable InGaAsP/InP Buried Rib Waveguide Vertical Coupler Filter`, Applied Physics Letters, Vol. 60, February 1992).
Typically, photonic integrated circuits are constructed on a planar platform. Therefore, in order to take full advantage of photonic integrated circuits, horizontal waveguide grating filters are preferred for on-chip monolithic integration with other photonic components. Horizontal planar filters comprising dissimilar waveguides by either different etch-depths or by different widths have also been demonstrated (for example see: Polarization-Independent Filtering in a Grating-Assisted Horizontal Directional Coupler`, S. Francois et al., IEEE Photonics Technology Letters, Vol. 7, July, 1997; see also: R. Maerz et al., `Spectral properties of asymmetrical optical directional couplers with periodic structures`, Optical and Quantum Electronics, Vol. 19, 1987). A drawback of the prior art devices is that the achieved dissimilarity for these waveguides is around 0.01, where dissimilarity is measured by the index differential, .DELTA.N=.vertline.N.sub.1 -N.sub.2 .vertline.. However, a .DELTA.N greater than 0.1 is typically desired because most practical applications require larger bandwidths and reduced crosstalk.